We have found that the induction of vitamin A deficiency in rats results in an adaptive response such that the flow of one-carbon compounds through the pool is primarily in the direction methionine synthesis rather than in the direction of carbon dioxide formation thus resulting in a redistribution of hepatic folate cofactors, notably a marked reduction of tetrahydrofolic acid. This adaptation occurs largely as the result of a marked decrease in N10formyltetrahydrofolate dehyrogenase and a marked increase in N5,10methylenetetrahydrofolate reductase, two key enzymes of the one-carbon pathway. We will first determine the time course of development of the effects of vitamin A deficiency on one-carbon metabolism and whether repletion with retinol or other retinoids will reverse the effects of the production of vitamin A deficiency. In addition we will determine if the effects of vitamin A deficiency on one-carbon metabolism which we have observed in liver are apparent in other rapidly dividing tissue such as testes, since studies from other laboratories have shown that testes are particularly susceptible to vitamin A deficiency. A second objective is to determine the metabolic reason for the marked change in enzyme activity in two key enzymes of the one-carbon pool due to vitamin A deficiency. Studies will be conducted to determine if actual changes in enzyme protein occur and/or if changes in levels of translatable mRNA coding for each protein occur, since studies by others have shown that genomic expression is altered as a result of vitamin A deficiency. To our knowledge, studies of this nature concerning the metabolic control of folate metabolism under any physiological condition have not been performed. Finally, our third major objective is to use the established methodology of tracer kinetic techniques to determine quantitatively the flow of one-carbon units through the folate dependent one-carbon pool under normal, physiological conditions and during the development of vitamin A deficiency. To date no quantitative measurements of flow through the pool have been made under any metabolic situation; therefore, the implementation of this methodology will significantly increase our ability to examine quantitative aspects of folic acid nutriture under a variety of situations.